In the past, T-shaped joint structures have been used for various structures such as vehicles, ships, buildings, bridges, and general industrial machinery. For example, a structure has been known including two side sills extending in the vehicle-longitudinal-direction at predetermined intervals in the vehicle-lateral-direction and a plurality of cross members connected to both of these side sills and extending in the vehicle-lateral-direction (see Japanese Utility Model Publication No. 60-124381).